1. Field of the Invention
The present invention relates to a tunnel unit for a scanning tunneling microscope (hereinafter referred to as an "STM") and, more particularly, to a tunnel unit for a microscope of this type that has a coarse adjustment mechanism in which the scanning head is supported by the use of screws at three points.
The present invention also relates to a scanning head used in a tunnel unit for an STM.
2. Description of the Related Art
Tunnel units for STMs that have a coarse adjustment mechanism, in which a scanning head having a probe mounted on a piezoelectric element is supported at three points by three screws provided through a specimen stage, are disclosed in, for instance, (1) J. Schneir, R. Sonnenfeld, O. Marti, P. K. Hansma, J. E. Demuth and R. J. Hamers, "J. Appl. Phys.", Vol. 63, No. 3, pp 717 to 721, 1988, and (2) O. Marti, B. Drake, S. Gould and P. K. Hansma, "J. Vac. Sci. Technol. A.", Vol. 6, No. 2, pp 287 to 290, 1988. Tunnel units according to these disclosures are shown in FIGS. 14A and 14B. As shown in these figures, each tunnel unit has a scanning head 110 and a specimen stage 120. The scanning head 110 is provided with a piezoelectric element 113 capable of serving as a fine adjustment mechanism, and a probe 115 mounted on the piezoelectric element 113. On the other hand, the specimen stage 120 is provided with three screws 112 whose ends project from the surface of the stage 120 on which specimens, such as a specimen 124, may be placed. The scanning head 110 is disposed upon the ends of the three screws 112. When the screws 112 are advanced or retracted by suitably rotating them, the space between the probe 115 provided on the scanning head 110 and the specimen 124 placed on the specimen stage 120 is adjusted. An opening 10a for observing the probe 115 is formed through a side portion of the scanning head 110. The adjustment of the space between the probe 115 and the specimen 124 is performed while the tip portion of the probe 115 is observed through the opening 10a and at a magnified scale achieved by using a magnifier.
However, the rigidity of the tunnel unit is inadequate if the scanning head 110 is simply disposed upon the three screws 112. To overcome this problem, the tunnel unit has, e.g., the following arrangement. That is, springs 129 are provided between the scanning head 110 and the specimen stage 120 to apply tensile force between these members, as shown in FIG. 14A, or spherical permanent magnets 119 are mounted on the ends of the screws 112 and, simultaneously, the scanning head 110 is formed of a magnetic body, as shown in FIG. 14B, thereby causing attraction by virtue of the magnetic force.
The conventional tunnel units, however, encounter the following problems.
In a tunnel unit in which the springs 129 are used to pull the scanning head 110 and the specimen stage 120 toward each other, the springs 129, which must be disassembled and reassembled each time the probe 115, the specimen 124 or the like is replaced, lead to inconvenience. Particularly when springs 129 having a large spring constant are used or when a plurality of springs 129 are used in order to enhance the rigidity of the tunnel unit, the disassembly and reassembly of the springs 129 may cause the scanning head 110 to become inclined or subjected to impact, often resulting in the probe 115 coming into contact with the specimen stage 120, the specimen 124, or other associated parts, thereby damaging the probe 115, the specimen 124, or the like.
Furthermore, a tunnel unit in which the spherical permanent magnets 119 are used involves the risk that, when the scanning head 110 is being disposed on the specimen stage 120, these members may be abruptly attracted to each other by the magnetic force, damaging the probe 115, the specimen 124, or the like. Also, because permanent magnet materials having a large coercive force, such as Alnico and ferrite, are in general hard and brittle, they can only be machined with difficulty. It is therefore difficult to machine such a material to assume a spherical configuration and, hence, to provide it with a low level of surface roughness at least at the part where it is to come into contact with the scanning head 110. Another problem is that, if a sintered magnet material is used, voids or cavities tend to be formed in the spherical surface after surface machining, while if permanent magnets in which a plastic material is charged with needle-shaped particles of an iron-cobalt alloy are used, they may encounter such problems as surface wear, a low level of rigidity, and a large coefficient of thermal expansion.
The formation of the opening 10a in the scanning head 110 involves the following problem. If any sound waves should propagate from the outside of the tunnel unit into the scanning head 110 through the opening 10a, they may directly propagate to the piezoelectric element 113. In such cases, there is a risk of electric noise being generated. An STM of the type being discussed employs tunnel current which is about 1 nA and is thus very small relative to the bias voltage applied between the probe 115 and the specimen 124, which is about 20 mV Therefore, if any electromagnetic waves should propagate from the outside into the scanning head 110 through the opening 10a, there is a risk of a great deal of electric noise being generated. Possible causes of electromagnetic waves entering the scanning head 110 include high voltage applied to the deflection electrode of a CRT combined with the STM to display, using the tunnel current, images of the surface of the specimen 124 for observation thereof. In order to prevent any sound waves or electromagnetic waves from entering, observation has hitherto been conducted, with the scanning head 110 covered with a case formed of a plastic or metal material. However, if the specimen 124 has to be observed while such a case is covering the scanning head 110, the efficiency of an observation operation deteriorates greatly.